Safe liquid source containers

ABSTRACT

Containers for providing vapor phase reactant from liquid sources include bubbler designs and designs in which carrier gas flows over the liquid surface. Among the bubbler arrangements, a bypass conductance is provided to release excess pressure from the gas volume inside the container, or an enlarged bubbler tube is provided with a volume sufficient to accommodate all possible liquid backflow without having the liquid exit the container. Among the overflow designs, flow dividers provide a tortuous path for the gas to increase the time exposure of carrier gas packets to the evaporating liquid surface. The flow dividers can be microporous to encourage capillary action, thereby increasing the evaporating surface. The tortuous gas flow path can be separated from the liquid phase by a breathable semi-porous membrane that permits vapor phase reactant to pass through but prohibits liquid from passing in the other direction.

FIELD OF THE INVENTION

The present invention relates generally to vapor source containers andmore particularly, to liquid source containers used for generating vaporprecursor to be used in downstream chemical reactions.

BACKGROUND OF THE INVENTION

Bubbler-type liquid sources are often used in chemical reactors, such assemiconductor processing tools. Examples include chemical vapordeposition (CVD) and atomic layer deposition (ALD) reactors. Bubblerstend to exhibit better efficiency than so-called “overflow” sources, inwhich vaporization depends on heating and the carrier gas simply flowsover the surface of liquid, rather than being bubbled through theliquid. The efficiency of the overflow sources depends greatly on flowconditions. On the other hand, bubbler type sources are not always safe.Under certain conditions there is risk that the pressure inside thesource bottle is higher than in the inlet feeding tube (which issubmerged in the liquid source). The higher pressure pushes the liquidprecursor material into the carrier gas inlet of the feeding tube. Thisuncontrolled situation is a safety and contamination risk. This type ofrisk is higher with ALD reactors due to the fact that the operation isbased on pulsing and there is typically more than one route for thegases into the reactor. These conditions increase the risk ofaccidentally pumping the gas inlet tubing to a lower pressure than theliquid container.

A source container 10 shown in FIG. 1 is positioned in atemperature-controlled environment 12. The container 10 has a bottleinlet 20 for the inactive carrier gas and a bottle outlet 22 for the gasmixture generated in the source container 10. A manual inlet isolationvalve 14 and an outlet isolation valve 16 are used for isolating thecontainer 10 from the surrounding conduits and room atmosphere when adepleted container is replaced with a refilled one. The source containerhas a refill port 18 that is used for adding liquid source chemical tothe container, and in some arrangements the port is provided in fluidcommunication with an auto-refiller to maintain a minimum level ofliquid source. A computer-controlled three-way valve 24 directs inactivegas either to the container 10 or to a by-pass line 90.

A computer-controlled source valve 26 is used for regulating the flow ofgas mixture from the container 10 to the reaction space 52. Acomputer-controlled by-pass valve 28 is used for purging the inlet 20and outlet 22 of the residual source chemical vapor before replacing adepleted container 10 with a fresh one. The by-pass valve 28 is keptclosed during deposition processes. The computer controlled valves 24,26, 28 are placed in a temperature-controlled environment 30 to preventthe condensation of the source chemical vapor at these valves.

A reaction chamber 50 defines a reaction space 52 in which a substrate54 is housed. The reaction space 52 is in fluid communication withchemical sources, including the liquid source container 10, through asource conduit 92, and in fluid communication with a vacuum pump 70through an exhaust conduit 72. The vacuum pump has an outlet 74 forgases. A back-suction conduit 96 is in fluid communication with theexhaust conduit 72 and in fluid communication with the source conduit 92at a junction 98 that is placed between the reactor 56 and thecomputer-controlled source valve 26. A back-suction restrictor 104, e.g.a capillary, restricts the flow rate of gases through the back-suctionconduit 96 to the exhaust conduit 72.

A by-pass restrictor 100, e.g., a capillary, restricts the flow rate ofinactive gas from the three-way valve 24 through the by-pass conduit 90that is connected to the source conduit 92 at a connection point 94. Theconnection point 94 is placed in the source conduit 92 between thereaction chamber 50 and the junction 98 of the back-suction conduit 96.Flow rate restrictors 106, 108, e.g., capillaries, are placed in thesource conduit 92 on both sides of the by-pass conduit connection point94 to form a gas diffusion barrier volume between the said flow raterestrictors 106, 108. The flow rate restrictors 100, 104, 106, 108 areshown inside temperature-controlled environments 30, 56 to enable fastpurging of the conduits 90, 92.

At least two source chemicals (one shown in FIG. 1) are connected to thereaction chamber 50 and are alternately pulsed into the reaction space52 during an ALD process. The source chemical pulses are preferablyseparated with inactive gas flow that purges the reaction space,although other means are known for keeping the reactant gases separatespatially and temporally. A typical ALD pulsing sequence consists offour basic process steps: reactant A pulse, purge A, reactant B pulse,purge B. The pulsing sequence is repeated as many times as is needed forobtaining a thin film of desired thickness. In other arrangements, thepulsing sequence can be more complicated.

Referring to FIG. 1, during a reactant A pulse, an inactive gas (e.g.,nitrogen or argon) flows from an inactive gas source 40 through a massflow controller 42. The three-way valve 24 guides the inactive gas tothe source container 10. Evaporated source chemical vapor diffuses withthe inactive gas inside the source container 10 and flows as a gasmixture to the outlet 22. The source valve 26 allows the gas mixture toflow to the reaction space 52 where the source chemical moleculeschemisorb on the substrate 54 surface until available reactive surfacesites have been filled with the molecules and the chemisorption processsaturates, leaving no more than one molecular layer of the sourcechemical molecules or their chemisorbed fractions on the surface.

During a purge A step, the source valve 26 is kept closed and thethree-way valve 24 guides the flow of the inactive gas through theby-pass conduit 90 to the gas diffusion barrier volume that is locatedin the vicinity of the connection point 94. The inactive gas flowdivides into two parts with the help of the flow rate restrictors 106,108. About 90% of the inactive gas flows to the reaction space 52 andpurges the residual reactant A away from the reaction space 52 to theexhaust conduit 72. About 10% of the inactive gas flows backwardlythrough the source conduit 92 to the junction point 98 of theback-suction conduit 96 and then the gas flows through the back-suctionconduit 96 to the exhaust conduit 72 and finally to the vacuum pump 70.The backward flow makes sure that source chemical molecules do notdiffuse along the source conduit 92 to the reaction space 52 during thepurge period.

It will be understood that certain problems related to the liquid sourceare created with the source pulsing method. If the source container 10is a bubbler and the three-way valve malfunctions, it is possible thatthe gas pressure at the inlet 20 to the container 10 becomes smallerthan the gas pressure inside the container 10. In that case the bubblerpressure tends to push some liquid toward the gas inlet 20 and evenfurther upstream of the container 10. The by-pass conduit 90 can becomethereby contaminated with the source chemical and the reactor does notoperate in ALD mode any longer.

As shown in FIG. 2 a, a bubbler tube 200 extends into the sourcecontainer 10 so that the tip 202 of the bubbler tube is near the bottomof the source container 10. The tip 202 is thus below the surface 220 ofthe liquid source chemical. During a source chemical pulse, inactive gasis fed to the inlet 20, flows through the manual inlet isolation valve14, through the bubbler tube 200 and forms bubbles 206 that rise to thesurface of the liquid 220. Source chemical molecules diffuse into theinactive gas, forming a gas mixture, and the gas becomes more or lesssaturated with the source chemical vapor. The gas mixture leaves the gasspace 208 of the container 10 through the manual outlet isolation valve16 and continues through the outlet 22 and further to the reaction space(not shown).

Referring to FIG. 2 b, in case of a malfunction, the pressure of theinactive gas at the inlet 20 may become smaller than the pressure of thegas mixture in the gas space 208 of the container 10. The pressurepushes the liquid surface 220 and forces liquid back through the bubblertube 200 so that the liquid surface 222 in the bubbler tube 200 creepstowards the inlet 20 and upstream areas of the source conduits. This isproblematic, especially in liquid sources that undergo rapid pressurevariations during ALD processing. A backlash valve (not shown) placed atthe inlet of the container is not entirely fail-safe because ofcorrosive source chemicals.

This problem has partially been alleviated with the use of an “overflow”source, shown in FIGS. 3 a and 3 b, rather than a bubbler, but itcreates certain new problems. Inactive gas flows through the inlet 20into the container 10. An inlet conduit is arranged to have its tip 300always above the liquid surface 220 so that back-flow of the liquidupstream of the source is not possible. The inactive gas mixes with thesource chemical vapor and the mixture enters a flow space 302 (arrangedcoaxially with the inlet in the illustrated example) and then themixture flows through the manual outlet isolation valve 16 through theoutlet 22 and further to the reaction space (not shown).

When the liquid is gradually consumed, the surface 220 of the liquidlowers and the distance between the tip 300 of the inlet conduit and theliquid surface 220 increases. When the distance becomes large, as shownin FIG. 3 b, it takes a longer time for the source chemical molecules tomix with the inactive gas and the gas mixture becomes more dilute asdeposition proceeds. It can be understood that the decreasing sourcechemical concentration creates problems with the dosage of the sourcechemical into the reaction space. Since the dosage per timed pulsedecreases over time, either an overlong pulsing period is employed,which would waste time and reactant during initial phases, or thepulsing period must be increased over time, which is impractical formanufacturers to implement in process recipes that should be consistentfrom run to run. Rather than risking underdosage, the tendency is tooverdose. ALD processes are not particularly sensitive to the dosage,since the surface reactions are self-saturating such that depositedlayers remain uniform despite overdosage. Nevertheless it may bedifficult to control the amount of overdosage of the source chemical andrather a lot of chemical is wasted.

Thus, there is a need for an improved liquid source that addresses atleast some of the problems described above.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a liquid source chemicalvaporizer is provided with a container configured to hold liquid sourcechemical up to a liquid fill level, and to additionally define an innergas volume. A carrier gas inlet communicates carrier gas into thecontainer. A gas outlet communicates with the inner gas volume of thecontainer. A porous element is positioned to be in contact with theliquid source chemical and in contact with the inner gas volume withinthe container.

In one embodiment, the porous element is a flow divider extending theheight of the container, defining a lengthened gas flow path in contactwith a surface of the liquid, and also increasing the evaporatingsurface by encouraging capillary migration of the liquid up themicroporous wall. In another embodiment, the porous element is asemi-porous membrane allowing vapor phase reactant to pass through, butnot liquid. In another embodiment, the porous element is a bubbler tube,wherein the pores provide a by-pass conductance for excess pressure inthe container to equalize pressure between the inlet and the inner gasvolume without forcing liquid back up the bubbler tube. In onearrangement, an elastic liner within the bubbler tube collapses uponbuild-up of excess pressure within the container, preventing liquid flowup the bubbler tube. In another arrangement, a restrictive capillaryprovides an alternative or additional by-pass conductance.

In accordance with another aspect of the invention, a liquid sourcechemical vaporizer is provided for vaporizing liquid source chemical anddelivering vapor phase chemical. The vaporizer includes anothercontainer that is configured to hold liquid source chemical. A carriergas inlet communicates with an inner gas volume defined within the outercontainer. A gas mixture outlet communicates with the inner gas volume.At least one flow divider defines a gas flow path through the inner gasvolume.

In one embodiment, the flow divider has microporous walls for increasingthe evaporation rate of the source chemical while avoiding the backwardflow of the liquid source chemical. In another embodiment, the at leastone flow divider includes a semi-permeable gas-liquid interface thatkeeps the flow conduit dry. The semi-permeable membrane can be a simpletube, or a part of a wall separating the liquid from an innercompartment having further flow dividers defining a labyrinth gas flowpath in communication with membrane.

In another aspect of the invention, a bubbler system is provided. Thesystem includes a container, a bubbler tube extending into liquid sourcechemical within the container, and a gas mixture outlet communicatingwith a gas volume above the liquid chemical. A by-pass conductance pathis provided for releasing excess pressure within the container relativeto the gas inlet.

In one embodiment, the by-pass conductance is provided by openings in abubbler tube providing conductance from an inner gas volume within thecontainer to the inlet, thereby by-passing the liquid phase. In anotherembodiment, the by-pass conductance is provided by a capillary tube incommunication with the inner gas volume and the inlet. In still anotherembodiment, an enlarged bubbler tube accommodates any possible backwardsliquid flow, sized such that the liquid level drops below the bubblertube opening before liquid rising up the bubbler tube can reach the gasinlet. Thereafter, gas can flow directly through the bubbler opening tothe inlet, relieving any additional excess pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects, features and advantages of the present invention willbecome apparent from the following description of the preferredembodiments and from the appended drawings, which are meant toillustrate and not to limit the invention, and in which:

FIGS. 4 a–12 are non-limiting illustrations (not to scale) of variouspreferred embodiments.

FIG. 1 is a schematic drawing of a prior art ALD system showing oneliquid source coupled to the reactor.

FIGS. 2 a and 2 b are schematic side views of a prior art bubbler-typeliquid source.

FIGS. 3 a and 3 b are schematic side views of a prior art overflow-typeliquid source.

FIG. 4 a is a schematic, cross-sectional plan view of a source chemicalcontainer constructed in accordance with an embodiment of the presentinvention.

FIG. 4 b is a schematic side view of the embodiment shown in FIG. 4 a.

FIG. 5 a is a schematic, cross-sectional plan view of another embodimentof the present invention.

FIG. 5 b is a schematic exploded side view of the embodiment shown inFIG. 5 a.

FIG. 6 is a schematic, cross-sectional side view of a bubbler withparallel gas conductance paths in the form of porous inlet tubeconstructed in accordance with another embodiment.

FIG. 7 is a schematic, cross-sectional side view of a bubbler with acapillary inlet conduit parallel to the main inlet conduit, creatingparallel gas conductance paths in accordance with another embodiment.

FIG. 8 a is a schematic, cross-sectional side view of a bubbler withflexible collapsing tube in accordance with another embodiment.

FIGS. 8 b and 8 c are schematic side views showing details of theflexible collapsing tube of FIG. 8 a.

FIG. 9 a is a schematic, cross-sectional side view of a liquid sourcewith an inner inlet container in accordance with another embodiment.

FIG. 9 b is a schematic side view of the liquid source of FIG. 9 a underthe influence of a high internal gas pressure.

FIGS. 10 a and 10 b are schematic side views of an embodiment modifiedfrom the embodiment in FIGS. 9 a and 9 b.

FIG. 11 is a schematic, cross-sectional side view of a liquid sourcehaving a semi-permeable flow conduit in accordance with anotherembodiment.

FIG. 12 a is a schematic, cross-sectional side view of a liquid sourcehaving a double bottom in accordance with another embodiment.

FIG. 12 b is a schematic cross-sectional plan view of the liquid sourceof FIG. 12 a, showing a flow labyrinth within the lower bottom section.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A number of embodiments are provided for a liquid source chemicalapparatus that provides a vapor form of the source while minimizing therisk of liquid travelling backwardly through a carrier gas inlet tube.Certain of the embodiments provide such safety in the form ofmodifications to conventional “overflow” type of liquid sources, whereinthe carrier gas tube is not submerged in the liquid, whereas otherembodiments provide bubblers with modifications to inhibit liquidbackflow through the carrier gas tube. Still other embodiments provide anovel structure employing semi-porous membranes for the carrier gas suchthat no route is provided for liquid flow out of the container duringnormal operation, and rather only vapor passes through the semi-porousmembrane. Several of the embodiments employ porous elements at theinterface between gas and vapor phases, advantageously enablingincreasing the surface area of the interface either by way of thesurfaces defined by the increased surface of the pores or by enabling alengthened gas flow path behind the porous element.

The liquid source chemical apparatus can be employed for providing vaporphase reactant to any suitable chemical reaction. The sources describedherein have particular utility for use with a vapor deposition reactor,such as chemical vapor deposition (CVD) or atomic layer deposition (ALD)reactors. As described in the Background section above, ALD isespecially susceptible to pressure fluctuations that risk sending liquidreactant up a bubbler tube. Accordingly, in one embodiment, a liquidsource chemical container is connected to an ALD chamber.

As is known in the art, atomic layer deposition employs alternatingsurface reactions whereby the apparatus, and particularly the substrateon which deposition is to be conducted, is maintained at a temperatureabove the condensation temperature for the vapor phase reactants andbelow the thermal decomposition temperatures. Sequential reactant pulsesare separated in time and space to avoid gas phase reaction, since thereactants used for ALD are typically highly mutually reactive. Forexample, after a first reactant pulse, excess reactant and anyby-product can be removed from the chamber prior to the next reactant,such as by purging with an inert gas. In a first pulse, reactantadsorbs, largely intact, on the substrate of interest in aself-limiting, self-saturating process, leaving no more than about onemonolayer of reactant. In a typical arrangement, the reactant includestermination ligands that are not reactive with the gas phase of the samereactant pulse. After removal of excess reactant, the next reactant willreact with the terminations, either stripping the ligands or replacingthe ligands with another element or molecule of interest for thedeposited layer. Excess of the second reactant is removed and either athird reactant can be introduced and removed, etc. or the cycle canstart again with the first reactant. Accordingly, no more than amolecular monolayer of the desired material is formed per cycle. Infact, typically less than a monolayer will form, due to steric hindranceby bulky source chemicals blocking possible reactive sites on thesurface or due to limited number of reactive sites on the surface.Variations on ALD, however, can arrive at more than a monolayer percycle, while still deriving some of the benefit of self-limitingbehavior.

As will be understood by the skilled artisan, the containers describedherein are typically provided with heaters to maintain a high vaporpressure within the container. Additionally, a vacuum is applied to thecontainer to improve the vaporization rate. Typical pressures inside thecontainer during operation are in the range of 5 mbar to 400 mbar, morepreferably between about 10 mbar and 200 mbar. The pressure inside thecontainer is higher than the pressure of the reaction chamber during adeposition process.

With reference to FIG. 4 a, according to one embodiment, an evaporatormade of porous material is placed inside a liquid source container 10.The container 10 is also referred to herein as an outer container,because some of the embodiments described hereinbelow include an innercontainer or compartment. As noted, low pressures and high temperaturesare typically employed to improve the vapor pressure of the sourcechemical, to provide vapor phase reactant to a downstream reactor.Accordingly, the outer container 10 is typically formed of a highstructural strength metal, such as stainless steel. Additionally, thematerial serves as a good conductor of heat energy, and can beexternally heated, e.g., by radiant heat. In other arrangements, theliquid source container can include an internal heater, such asresistive heaters.

FIG. 4 a shows a top view of a gas flow divider formed by a microporouswall 400 shaped like a spiral. Preferably, the porosity of the wall 400is in the range of 20%–80%, and more preferably 30% to 70%. In addition,or alternatively, the material of the wall 400 can be selected to beabsorbent (possessing high surface energy) to facilitate capillaryaction to cause the liquid to ride up the wall 400. In the illustratedembodiment, the spiral wall 400 extends a height 406 of the container10, from the bottom to the ceiling of the container 10. Inactive gas isfed approximately to a middle inlet point 402 of the container 10,proximate the container's ceiling. The gas flows towards an outlet point404 also near the container ceiling, although the positions of inlet andoutlet can be reversed. As shown in FIG. 4 b, manual valves 14, 16 areused for isolating the contents of the source container 10 from thesurroundings during the replacement of the container 10 with a freshlycharged container.

The wall 400 serves as a gas flow divider to lengthen the flow path ofthe carrier gas so that there is sufficient time to saturate theinactive gas with the source chemical molecules. The flow dividerlengthens the carrier gas flow path in contact with the liquid source bygreater than 3 times, and more preferably greater than 5 times thelength of a conventional “overflow” type of liquid source container. Theskilled artisan will readily appreciate that the conventional “overflow”liquid source provides only the distance from an inlet on the ceiling toan outlet on the ceiling, both in communication with a common gas space,for the gas to travel, with some possible diffusion to deviatetherefrom. Despite the porosity of the microporous wall 400, the carriergas naturally follows the path of least resistance such that the bulk ofthe inactive gas flows along the illustrated spiral flow path, graduallybecomes saturated with source chemical vapor, and exits the container 10at the outlet point 404 near the ceiling. The evaporation rate of theliquid is enhanced with the microporous and/or absorbency of thematerial of the wall 400 that acts as an evaporator. The microporouswall 400 above the liquid is wetted by the capillary forces that pullliquid upwardly through the open pore structure of the evaporator.

The height of capillary rise can be calculated with an equation h=2τ cosα/Rρg, wherein h is the maximum height to which the liquid surfacerises, τ is the surface tension of the liquid, α is the contact anglebetween the liquid surface and the pore (or the capillary) wall, R isthe radius of the pore, ρ is the density of the liquid and g is thegravitational acceleration. Pore surfaces with low surface energy repelliquids so pore surfaces with high surface energy are favored when highcapillary rise is desired. Materials with metal oxide surfaces, such asSiO₂ or TiO₂, have high surface energy, while materials withcarbon-containing molecular groups on the surfaces, such as —CH₃ or—CF₃, have low surface energy. Thus, porous quartz (SiO₂) attractsliquids and porous teflon (polymerized fluorinated hydrocarbon, e.g.Gore-Tex®) repels liquids. Making the pores smaller also increases thecapillary rise. In case of water, the height of capillary rise is about150 mm when the pore radius is 100 μm and the porous material mostlyconsists of SiO₂. Decreasing the pore radius to 30 μm increases theheight of capillary rise to about 500 mm. Thus, a nominal pore radius of10 μm–200 μm is preferred to maximize effective liquid surface areaexposed to the carrier gas flow, more preferably 30 μm–70 μm.

The evaporator inside the liquid container 10 can be constructed like aheat radiator. Part of it is always wetted with the liquid sourcechemical. There can be additional lamellas that will further enhance theevaporation of the precursor material. The gas flow is forced betweenthese lamellas. This will increase the evaporation surface, and moresignificantly the time of exposure of any given packet of carrier gas tothe liquid surface. The efficiency of the liquid source is improvedbecause of the improved evaporation rate and the concentration of thesource chemical vapor in the inactive gas will be relatively constantover time.

According to another embodiment modified from that of FIGS. 4 a and 4 band shown in FIGS. 5 a and 5 b, the evaporator includes a plurality ofconcentric microporous cylinders 500 that extend along container height504 from the bottom to the ceiling 510 of the container 10. Thecylinders 500 are preferably formed of a material having a porositybetween about 20% and 80%, more preferably between about 30% and 70%.The material can alternatively, or additionally, exhibit absorbency(possessing high surface energy) to facilitate capillary action by theliquid. The cylinders 500 each have about 5–20 mm wide slits 502 thatextend a slit height 506 from near the bottom edge of the cylinders tonear the top edge of the cylinders 500. As shown, the slits 502 ofadjacent cylinders 500 are arranged on opposite sides in staggeredfashion, such that gas must flow around the annular pathways definedbetween cylinders 500 to reach successive slits 502. Thus, the pluralityof concentric cylinders 500 define a tortuous path from the gas inlet402, opening above the liquid surface, to the gas outlet 404, also abovethe liquid surface. Preferably the path length is characterized by aratio between the path length and the total liquid surface area(length:area) between about 1: 1000 cm⁻¹ and 2: 10 cm⁻¹, more preferablybetween about 2: 100 cm⁻¹ and 1:10 cm⁻¹.

In operation, capillary forces lift liquid source chemical up themicroporous and/or absorbent cylinder walls. Inactive gas is fedapproximately to the middle point 402 of the container ceiling 510. Thegas flows a tortuous path between the cylinder 500 walls and through theslits 502 and gradually becomes saturated with vapor phase sourcechemical molecules as it traverses the tortuous path. Finally the gasmixture exits the container 10 at the outlet point 404 near the outeredge of the container ceiling 510. The source is safe to operate becausethe tip of the inlet conduit is arranged to be always above the liquidsurface. The evaporator structure enhances the evaporation of the liquidand the concentration of the source chemical stays constant in the gasmixture at the outlet 22 during the depositions.

According to still another embodiment shown in FIG. 6, a by-passconductance is arranged for releasing the pressure difference between afeeding tube inlet 20 and a gas space 208 inside the source container.The feeding tube 600 extending from the inlet 20 into the container 10is preferably made from a porous material. The pores (or other openings)are made more restrictive than the main opening 602 at the bottom of thefeeding tube 600, including accounting for the pressure of a “full” orfreshly refilled liquid. Preferably, the porosity is between about 30%and 70%, more preferably between about 40% and 60%, or the gas flowconductance through the feeding tube wall is about 1/20–1/5 of the gasflow conductance along the feeding tube, and can include macroperforations. Accordingly, in operation the main part of the inactivegas flow goes along the feeding tube 600 to the bottom opening and aminor part of the inactive gas flow goes through the porous wall of thefeeding tube 600. Preferably greater than about 50% of the gas flowexits at the bottom opening 602, more preferably greater than about 95%.Thus, the container operates essentially in bubbler mode during eachsource chemical pulse.

When the pressure at the inlet 20 becomes lower than the pressure of thegas space 208 of the container 10 above the liquid 204, gas flowsbackwardly through the walls of the porous feeding tube 600 into theupper part of the feeding tube. Liquid 204 stays in the container 10because the hydrostatic pressure of the liquid column inside the feedingtube will be about equal to the pressure difference across the walls ofthe porous feeding tube wall 600. The gas flowing through the porouswalls of the feeding tube 600 releases any overpressure before thepressure difference is big enough to push the precursor liquid backwardsto the inlet 20. Thus, the pores of the feeding tube 600 serve as aby-pass conductance for releasing any overpressure in the gas space 208.

According to Boyle's Law (pV=nRT) the quantity of the gas that must bereleased depends on the pressure and volume when the temperature isconstant. This means that the releasing flow should be higher when thebottle is almost empty compared to the situation where the bottle isfull. The effect is linear. The conductance of the porous tube is alinear function of the length of the tube. Since the risk of backflowincreases as the gas volume 208 increases, the growing capacity forbackflow release through the lengthening bare section of the porous tubecompensates for this increasing risk as the liquid 204 is consumed. Theliquid 204 is a liquid source chemical that is selected for example fromalkylaluminum compounds such as trimethyl aluminum (CH₃)₃A1, also knownas TMA, metal halides such as titanium tetrachloride TiCl₄, boroncompounds such as triethyl boron (CH₃CH₂)₃B, also known as TEB, siliconcompounds such as trisilane Si₃H₈, alcohols such as ethanol, and waterH₂O.

Referring now to FIG. 7, according to still another embodiment aparallel capillary line is arranged to prevent the back-flow of liquid.FIG. 7 shows a schematic side view of a bubbler source container 10having a by-pass line 704. The by-pass line 704 communicates with theinternal gas space 208 and equalizes the gas pressure between therestrictions or capillaries 700, 702 and the gas space 208 of thecontainer 10. The capillary 702 has high enough conductance to preventthe back flow of the liquid along the bubbler tube 600. On the otherhand, the capillary has low enough flow conductivity so that during asource pulse most of the inactive gas will flow from the inlet 20 alongthe bubbler tube 600 to the liquid 204, rather than directly to the gasspace 208. Preferably, during normal operation, greater than about 90%of the inert carrier gas flows through the feeding tube 600 and lessthan about 10% through the by-pass tube 704, more preferably less thanabout 5%. The bubbler tube 600 can be, for example, a metal tube, aperforated metal tube or a porous tube as described with respect to FIG.6. Flow conductivity through the wall of the bubbler tube 600 is addedto the flow conductivity of the capillary 702 to determine how muchcarrier gas flows directly to the gas space 208. Accounting for the gasflow escaping through the by-pass line 704 and through the pores of thebubbler tube 600 directly into the gas space 208, preferably greaterthan about 80% of the total inert carrier gas flow exits the mainopening 602 submerged in the water, more preferably greater than about90%. When the flow conductivity of the capillary 702 is increased theflow conductivity through the wall of the bubbler tube 600 can bedecreased.

A liquid column rising inside the bubbler tube 600 creates a hydrostaticpressure that pulls the liquid column downwardly. A general formula forthe hydrostatic pressure is presented in Eq. 1,p=gρh  Eq. 1

wherein p is hydrostatic pressure of the liquid, g is gravitationalacceleration, ρ is the density of the liquid and h is the height of theliquid column measured from the liquid surface 220. Any pressuredifference between the capillary space (between capillaries 700, 702)and the gas space 208 of the container 10 lifts liquid up along thebubbler tube 600 until the pressure difference is equal to thehydrostatic pressure of the liquid column. Because the pressuredifference is eliminated by gas flow through the capillary 702, theliquid column will also be eliminated and the liquid surface levelinside the bubbler tube 600 will be about the same as the liquid surfacelevel 220 within the container 10.

According to another embodiment shown in FIGS. 8 a, 8 b and 8 c, theporous bubbler tube 600 is complemented with a very elastic inner tube800. The bubbler tube 600, which can be made, e.g., of metal, polymer orceramic material, supports the elastic tube 800. If a pressuredifferential urges the liquid to flow backwardly to the bubbler tube600, the elastic tube 800 collapses temporarily because of fluid flow802 through the porous or perforated wall of the bubbler tube, and thebackflow stops. The elastic tube stays collapsed as long as there ishigher pressure in the liquid 204 than at the upper end of the bubblertube 600. Preferably, the elastic tube 800 has a Shore hardnessaccording to DIN 53505 A between about 30° and 70°, more preferablybetween about 35° and 45°.

Referring now to FIGS. 9 a and 9 b, according to still anotherembodiment, the feeding tube is enlarged relative to a conventionalbubbler tube. This enlarged bubbler or inlet tube is referred to for theillustrated embodiment as an inlet container 900, defining an innerspace 902 with a volume V_(I). The bottom of the inlet container 900 isopen. The source container 10 volume V_(S) and the liquid fill level 220of the container 10 are selected so that the volume V_(L) 904(representing the amount of the liquid 204 between the “full” liquidsurface level 220 and a bottom edge 906 of the inlet container 900), issmaller than the volume V_(I) of the entire inlet container 900.

While the depth of the tank below the bottom edge 906 of the inletcontainer 900 is irrelevant to this function, typically the inletcontainer 900 should reach near the bottom of the container 10 in orderto function as a bubbler until the liquid 204 is nearly exhausted, forthe sake of efficiency. Accordingly, the volume 902 of the inletcontainer 900 preferably represents greater than about 15% of the volumeV_(s) of the source container 10 (including both the liquid volume 204and the gas volume 208), and more preferably the volume V_(I) is greaterthan about 40% of the container volume V_(s). Without regard to theoverall height or depth of the container 10, the inlet container 900 ispreferably designed to have a horizontal cross-sectional surface arearepresenting greater than about 40% the horizontal cross-sectionalsurface area of the source container 10 (also referred to as an outercontainer), more preferably greater than about 50%.

In operation, possible back flow of liquid is stored into the volumeV_(I) of the inner space 902, as shown in FIG. 9 b. Flow of the liquidto the inlet container 900 stops when the liquid surface level 220reaches the bottom edge 906 of the inlet container 900. After that onlygas can bubble 910 through the inlet container 900. Accordingly, thebottom edge 906 of the inlet container 900 provides a by-passconductance path 912 for gas to bypass the remaining liquid below theinner container 900, thus releasing any pressure difference between thefeeding tube inlet 20 and the gas space 208 within the container 10.Disastrous flow of the liquid source chemical upstream of the inletcontainer 900 to the inactive gas feeding system is prevented.

Modification of the embodiment presented in FIGS. 9 a and 9 b results inan inlet container design that has certain benefits. FIGS. 10 a and 10 bshow an inlet container 900 that has a reduced or restricted opening1000 against the liquid phase 204. The overall volume of the inner space902 of the inner container 900 is preferably as described above withrespect to FIGS. 9 a and 9 b. The restricted opening 1000, however,preferably represents less than 10% of the total horizontalcross-sectional surface area of the inner container 900, more preferablyless than about 5%. Inactive gas has a high flow speed through therelatively small opening 1000 and the bubbles 206 will be smaller thanin case of a larger opening. A coarse sinter can also be placed at theopening 1000 to even further decrease the size of bubbles. Smallinactive gas bubbles saturate with source chemical molecules faster thanlarge bubbles, because small bubbles have a higher surface area:volumeratio than large bubbles.

According to still another embodiment shown in FIG. 11, the liquidsource is constructed so that the inactive carrier gas flows along asemi-permeable tube 1100 that is at least partly submerged under theliquid surface level 220. The semi-permeable membrane of the tube 1100serves as a flow divider for guiding gas flow and as the evaporatingsurface, since only vapor form of the source chemical passes from theliquid phase 204 into the tube 1100, as indicated by the arrows 1102.Desirably, the tube 1100 is elongated and follows a relatively long paththrough the liquid 204 to increase the overall surface area availablefor evaporation. Source chemical molecules that have diffused throughthe tube 1100 become mixed with the inactive gas so that a saturatedmixture is obtained inside the tube 1100 in the source container 10 bythe time the gas mixture reaches the outlet. Thus, no route exists forliquid source chemical 204 to escape the container 10 during normaloperation.

The semi-permeable tube 1100 is made, for example, of a Gore-Tex®membrane supported with a perforated polymer tube. A Gore-Tex® membranecomprises a polytetrafluoroethylene (PTFE)-based expanded fabric thathas billions of pores per square inch. The pores are too small (10–30μm) for liquid droplets to pass through, so liquid stays on the outside.On the other hand the pores are large enough to allow molecules in thevapor formed from the liquid to pass through the membrane by diffusion,making the membrane “breathable”. The membrane has low surface energy sothat the liquid 204 does not wet the membrane. The surface tension ofthe liquid keeps the liquid outside the tube 1100. Gore-Tex® iscommercially available from the W. L. Gore Corp., Newark, Del.Preferably the porosity of the semi-permeable membrane of the tube 1100is between about 70% and 93%, more preferably between about 80% and 85%.According to still another embodiment the semi-permeable tube 1100 ismade of a hydrophilic non-porous polymer such as hydrophilic polyesterblock copolymer commercially sold under trademark Sympatex®. Moleculeshaving polar functional groups such as water and amines travel from theliquid phase through the hydrophilic zones in the Sympatex® membrane andenter the gas phase. Liquid droplets cannot flow through the non-porousmembrane.

With reference now to FIGS. 12 a and 12 b, the evaporating surface canbe constructed as an interface between the liquid phase 204 and an innercompartment 1200. The inner compartment 1200 is at least partiallysubmerged within the liquid 204 and comprises a perforated wall 1208with an adjacent semi-permeable membrane 1210 (e.g., Gore-Tex®)supported either on the inner container side or on the liquid side ofthe perforated wall 1208. In the illustrated embodiment, the perforatedwall 1208 and the semi-permeable membrane 1210 serve to create a doublebottom for the container 10. In addition to providing the evaporatingsurface, the membrane 1210 also serves as a flow divider, partiallydefining a gas flow path.

In other arrangements, the inner compartment 1200 can be formed on aside wall, such as in the form of an annular space between the outerwall of the container 10 and a perforated inner cylindrical or othershaped container defined by a perforated wall 1208 and attached membrane1210. In still another arrangement, a container is completely surroundedby and submerged within the liquid 204, in which case multiple or allwalls of the inner compartment can be formed by perforated walls andattached membranes. In this example, a particularly large surface areaprovides the evaporating surface for liquid to vaporize through themembrane into the inner compartment 1200.

Preferably, the inner compartment 1200 defines a tortuous gas flow pathfrom the container's gas inlet to outlet, interfacing with the liquid204 only by way of the membrane(s). As shown in FIG. 12 b, a gas flowlabyrinth within the inner compartment 1200 can be machined, forexample, from a block of metal. Circular gas flow channels are separatedfrom each other by additional flow dividers in the form of walls 1220that extend a height 1206 from the bottom to the wall 1208 of the innercompartment 1200. Openings 1222 are machined into the walls 1220,preferably on alternated sides, so that gases flow along a tortuous pathfrom one circular channel to the next one until the exit point 1204 ofthe inner compartment 1200 is reached. Inactive gas is fed to the inletpoint 1202 of the inner compartment 1200 and the gas gradually becomessaturated with the source chemical molecules that have diffused from theliquid phase 204 through the membrane 1210 on the perforated wall 1208to the gas flow paths defined within the labyrinth. It will beunderstood that the labyrinth within the inner compartment 1200 can takeother more or less complicated forms, such as the spiral flow divider ofFIG. 4 a. The membrane 1210 acts as an evaporator surface that does notlet any liquid droplets through. One benefit of the construction is thatthe gas flow channels stay dry during the deposition processes.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt to a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A liquid source chemical vaporizer for vaporizing liquid sourcechemical and delivering vapor phase chemical, comprising: a containerconfigured to hold liquid source chemical up to a liquid fill level andto additionally define an inner gas volume; a carrier gas inletcommunicating carrier gas into the container; a gas outlet communicatingwith a vapor deposition reactor and the inner gas volume of thecontainer; and a porous element comprising at least one openingpositioned to be in contact with liquid source chemical and a pluralityof openings positioned to be in contact with the inner gas volume withinthe container.
 2. The source chemical vaporizer of claim 1, wherein thecarrier gas inlet comprises a bubbler tube extending through the innergas volume into the liquid source chemical, the porous element being thebubbler tube, whereby the inner gas volume is above the liquid filllevel and communicates gas through pores in the bubbler tube.
 3. Thesource chemical vaporizer of claim 2, wherein pores in the bubbler tubeare sized to produce a flow rate to the inner gas space above the liquidfill level, under normal operation, of greater than about 50% of a flowrate of carrier gas through a bottom of the bubbler tube producingbubbles within the liquid source chemical.
 4. A liquid source bubblersystem, comprising: a container configured to hold liquid sourcechemicals; a bubbler tube communicating with an inert gas source, thebubbler tube extending into an opening within a liquid storage space; agas outlet communicating with a vapor deposition reactor and an innergas space defined within the container above the liquid storage space;and a by-pass conductance route through the bubbler tube configured torelease excess gas pressure from within the inner gas space, therebyinhibiting liquid flow up the bubbler tube.
 5. The source chemicalvaporizer of claim 1, wherein the material of the porous element ismicroporous.
 6. The source chemical vaporizer of claim 1, wherein theporous element is a porous feeding tube.
 7. The source chemicalvaporizer of claim 6, wherein the porous feeding tube is configured suchthat hydrostatic pressure of a liquid column within the porous feedingtube is about equal to a pressure difference across walls of the porousfeeding tube.
 8. The source chemical vaporizer of claim 6, wherein poresof the porous feeding tube serve as a by-pass conductance for releasingoverpressure in the inner gas volume.
 9. The source chemical vaporizerof claim 1, wherein the porous element has a porosity between about 30%and 70%.
 10. The source chemical vaporizer of claim 1, wherein theporous element has a porosity between about 40% and 60%.
 11. The sourcechemical vaporizer of claim 1, wherein the porous element defines thegas flow path within a porous feeding tube having a conductance that isa linear function of a length of the tube.